Based on the few facts, and your comment that the executor named in the will is this fraudulent daughter, you can take certain actions. The first should be to immediately consult with counsel to review the will and other circumstances (such as, were there any prior will which did not name this "daughter" and was the deceased in her right mind as of the date that this current will was allegedly drafted? Some family member can and should petition in court to open the estate and appoint an executor, while at the same time attacking the validity of the current alleged will--and if there is a validly written and signed previous will, you can ask the court to enforce that one. If no prior will, the estate will pass by statutory intestate succession. This sounds as though the false "daughter" may very well litigate this matter, and a judge will have to decide where the truth lies. There are criminal elder abuse laws, as well as for fraud, forgery and theft, and if this person faked the will in order to seize the assets wrongfully, she can be prosecuted.
It depends on who holds the title/deed to the house? Who is legally entitled to reside in that house?
The will names an executor. The executor is the gatekeeper/custodian/guardian of the property in the estate. The executor should hire an attorney to assist him/her immediately! Good luck.
Attorney Urba has provided you with solid advice. If the alleged daughter and the grandmother held title to the home as "joint tenants with right of survivorship," then the property is hers (absent proof of fraud or undue influence in the execution of the underlying deed). If the deed was solely in the name of your son's grandmother, then the designated executor should probate the will with the assistance of an experienced estate administration attorney. The attorney will advise the executor as to the alleged daughter's rights under the will and what proof she may need to establish her claim to be a daughter. Good luck to you and your family.
This information is presented as a public service. It should not be construed to be formal legal advice nor considered to be the formation of a lawyer/client relationship. I am licensed in Connecticut and New York and my answers are based upon the law in those jurisdictions. My answer to any specific question would likely be different if I were to review a client's file and have the opportunity to interview the client. Accordingly, I strongly urge you to retain an attorney in your jurisdiction with respect to any legal matter.
Your son should consult an attorney to advise him of his rights. If he is not named in the will, he may have no claim to the estate. If the will leaves everything to a named individual, even if she is not legally a daughter, and the will is valid, that person will inherit the estate.
The above answer is not to be considered legal advice and should not be relied upon as such. You should consult your attorney for specific legal advice as to your individual situation.